TA Final Review. Class Announcements. Objectives Today. Compare True and Apparent brightness. Finding Distances with Cepheids

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1 Class Announcements Vocab Quiz 4 deadline is Saturday Midterm 4 has started, ends Monday Lab was in the Planetarium. You still need to do the 2 questions Check PS100 webpage, make sure your clicker is registered. TA Final Review Final Review TBA How much of the book would you estimate that you have read this year? a) % b) 60-80% c) 40-60% d) 20-40% e) 0-20% Objectives Today Distance Scales Cepheid Variables & Supernovae Understand structure of Milkyway Identify different types of galaxies Cosmological redshift Hubble Law Big Bang and evidence for it Estimate distances to galaxies Compare True and Apparent brightness Some stars, called Cepheids,, are very bright and unstable, pulsating with periods that are proportional to their true brightnesses. Finding Distances with Cepheids We can measure a Cepheid s s period and apparent brightness, and then infer the absolute brightness from the period-luminosity relationship. With the apparent and absolute brightness we can then get a distance to the star. This is only good for finding distances to the nearest 100 galaxies. You have to resolve the star to get the period. 1

2 What is the Milkyway? We know we are in a disk, band of stars across the sky. The sun is 2/3 the way from the galaxy center to the edge. Globular clusters were used to find the center of the galaxy. Variable stars were used to determine distances The Sun x Images of the Milkyway Globular Clusters x External Galaxies Galaxies: Huge Collections of Stars In 1920 the Shapley-Curtis debate ensues Planetary systems forming? External Galaxies? In 1923 using Cepheid variable stars, Edwin Hubble determined that the Andromeda galaxy was a separate system of stars. Soon it was realized that all spiral-shaped shaped nebula were galaxies of stars similar to the Milky Way. On average over 100 billion stars per galaxy! Huge! The universe itself is only 10 5 times larger than a typical galaxy. At least 50 billion of them. Galaxies fall into four main classifications according to their shapes. First Sketch of a galaxy. William Parsons, 1845 Large galaxies, red stars, no blue stars Ellipticals What does the absence of blue stars tell us? A. That there isn t t enough gas in these galaxies to make large stars B. That the galaxy is moving towards us C. That the galaxy is old D. That there aren t t any new stars forming 2

3 Gas, dust lanes, blue stars, young star forming regions Spirals Barred Spirals Irregular/Peculiar Gas, dust lanes, blue stars, young star forming regions, no definite shape Galaxy Clusters Galaxies are held together in small to large groups What holds them together? What force holds galactic clusters together? a) The weak nuclear force b) The strong nuclear force c) The electromagnetic force d) The gravitational force Cosmological Redshift Each atom emits at characteristic wavelengths (spectral lines) Certain patterns of prominent lines are observed in the spectra of all stars & galaxies E.g. hydrogen In the early 1900 s, it was found that wavelengths measured for distant galaxies do not match laboratory measurements Distant galactic lines are always shifted to longer wavelengths True for all galaxies but the few closest. Why? 3

4 Doppler Shift The Doppler effect causes waves to shift up to a higher frequency as they move towards you and down to lower frequency as they move away Sound examples race car engine train whistle Light examples cosmological redshift If all the galaxies are moving away, why are some of the galaxies close to the Milkyway blueshifted? a) They are spiral galaxies with lots of new stars b) Something is sucking nearby galaxies in c) I don t t know Hubble Law Hubble found that the speed at which a galaxy moves away from us is proportional to its distance. The farther away the galaxy is, the faster it recedes. Could this mean we are at the center of the universe? The Expanding Universe The farther away two galaxies are, the faster they move apart All galaxies appear to be moving away from all other galaxies. Expansion Distances from the Hubble Law We can use the Hubble Law to find distances to galaxies The larger the redshift,, the faster the velocity, the faster the velocity, the greater the distance. 4

5 The Big Bang The observed expansion of the universe has led to only one viable cosmological theory: The Big Bang. The universe began as a great infusion of pure energy about 14 billion years ago. Space expanded from that point in time. The energy cooled into matter. Evidence Supporting the Big Bang We see the expansion We see a faint glow left from the original fireball. Called the Cosmic Microwave Background or 3 3 K Background Radiation. Evidence Supporting the Big Bang It correctly predicts the chemical abundances of gas clouds. 75% hydrogen, 25% helium, a tiny amount of lithium. Fusion early after the big bang would produce that ratio of H to He. Big Bang Stellar Fusion Supernova 3 min 35 min Steps in Big-Bang Bang Theory (Interest only, not on test) Time Temp Structure sec sec sec 5x10 5 yrs yrs huge x All energy Elementary particles (electrons, quarks, etc.) Protons and neutrons Nuclei of 3 H, 3 He, 4 He, 5 Li Nuclei of 2 H Electrons attach to nuclei to form atoms Stars, galaxies, people Good Theories Give Good Predictions. What Does the Big Bang Predict About the Future? Gravity always pulls, never pushes. So the expansion must slow down unless there is another force pushing out. The Ultimate Fate of the Universe Not enough mass open universe Just right flat universe Too much mass closed universe (big- crunch?) Another force runaway universe. 5

6 Accelerating Expansion? Recent work compared distances measured two ways: From the apparent brightness of a known kind of supernova From the amount of cosmological redshift At large distances, the two methods do not agree (the supernova method gives bigger distances than the Hubble Law) This and other data are taken as evidence that the expansion is speeding up! None of the four known forces could cause this What Do We Observe? The expansion is accelerating! A runaway universe No one knows why No known force could cause this 6